Late neogene geology of the Whetukura area, Dannevirke region, North Island, New Zealand

Abstract

Neogene lithostratigraphic units and structural features of the Whetukura area at the eastern margin of the Dannevirke Basin are described and the stratigraphic relationship of these units and structural development of the area are determined. These are related to tectonic models for the development of the forearc region of the Hikurangi subduction zone. A geological map of the Whetukura area was compiled by means of field mapping, supplemented by the use of aerial photographs. Neogene strata unconformably overlying Upper Jurassic to Lower Cretaceous Torlesse rocks consist of a sequence of sandstones and mudstones with interbedded limestone bands. The age of the basal unconformity in the area is 10.5Ma, although in two exposures a younger basal conglomerate is found. Tongaporutuan age sandy mudstones and sandstones of the Mapiri Formation (10.5-6.0Ma) were deposited in an inner- to mid-shelf environment. Kapitean to Opoitian age conglomerates and sandstones of the Mangatoro Formation (6.0-5.0Ma) were then deposited in an inner-shelf to outer-shelf or deeper environment. Waipipian age Whetukura Limestone (3.6-3.1 Ma) and Mangapanian age Te Onepu Limestone (3.1-2.4Ma) were deposited in a near-shore, tide-dominated setting on the margins of a paleo-seaway that extended from Wairarapa to Hawke Bay during the Pliocene. Sandy mudstones of Raukawa Mudstone (3.6-2.4Ma) were deposited conformably between these two limestones in a predominantly mid-shelf environment of deposition. Cyclic mudstone and detrital limestone bands of Kumeroa Formation were then deposited during Upper Nukumaruan (1.8-1.2Ma) glacio-eustatic sea level changes. Mudstones and conglomerates of Castlecliffian age Mangatarata Formation (1.2-0.4Ma) were deposited conformably over the Kumeroa Formation. Geological structures are dominantly north-northeast striking reverse faults and associated east verging asymmetric folds. Faults characteristically exhibit rapid changes in dip separation along their lengths and are usually of limited length, commonly dying out along strike. The overall strike of reverse faults in the area is between 015 and 045, with an average of 030. Folds commonly affect the entire Neogene sequence and have long, west-dipping limbs and short, commonly faulted, east-dipping limbs. Generally, the east-dipping limbs steepen against the reverse faults. Steeply inclined to upright axial planes and the subhorizontal plunge of fold hinges in the area indicate horizontal shortening components. The axial planes of folds strike between 007 and 033. At least four episodes of deformation are identified Peneplaination of Torlesse Supergroup rocks occurred prior to the Tongaporutuan Stage (10.5Ma). A short period of deformation occurred during the Kapitean Stage (6.0-5.0Ma) with the development of thrust faults. A period of quiescence then followed (~6.0-2.4Ma) allowing sedimentation. This was followed by an episode of deformation during the 2.4Ma to 1.8Ma period, which resulted in folding of the sedimentary sequence into gentle to open asymmetric folds. Re-activation of pre-existing thrust faults then occurred. These were rotated 15 to 20 degrees westward to achieve steeper dips of 35 to 50 degrees. A subsequent period of quiescence lasted from 1.8 to 0.4Ma, followed by another period of folding and reverse faulting that was initiated at 0.4Ma and continues to the present. The re-activated faults were rotated westward again to achieve their present dips of 50 to 65 degrees. The average dip separation rate for 2.4-0Ma ranges from 0.1 to 0.4mm/a, 0.2mm/a being the dominant rate. Total shortening values for the Neogene sequence across the area range from 18 to 33%, with an average of 24%. Total shortening rates range from 3 to 5.5%/Ma, with an average of 4%/Ma. The Dannevirke-Ruataniwha Basin/Whetukura areas take up some of the plate normal component of the Australian-Pacific Plate convergence. Time varying deformation over the region is indicated by the difference in age of deformation with nearby areas. The age of the basal unconformity youngs towards the west from the Ruahine Range to east of the Whetukura area, suggesting that the areas could have behaved as crustal blocks, with associated migration of tectonic events. The nature of deformation in the Whetukura area indicates these block motions occurred episodically over time.